Your regexp can be simplified to the following: 

/\x26\x67(?:(?!\x26\x67).){7}./s

[download]

If you'd rather have simplicity at the cost of an extra check, use the following: 

/\x26\x67(?:(?!\x26\x67).){8}/s

[download]

/(?:(?!$re).){8}/
is the equivalent of
/[^$chars]{8}/
but it (negatively) matches entire regexps instead of a choice of characters.

In reply to Re^2: regex for negating a sequence by ikegami
in thread regex for negating a sequence by spurperl

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